Electrostatic scanning



Aug. 25, 1942. T R. E. GEAHAM 2,294,115

' ELECTROSTATIC SCANNING Filed June 4, 1941 4 Sheets-Sheet 2 ==642 37 50HP 60 46 llilllgl x 29 v 1 l 5/ T 30 511 LP 54 22% INVENTOR R. E. GRAHAMA 7'7'ORNEY LP lFrm I INVENZ'OR M W REGRAHAM ff 9 1 W T 4.2 4/

g- 1942- R. E. GRAHAM I 2,294,115

ELECTROSTATIC SCANNING Filed June 4, 1941 4 Sheets-Sheet 3 25 26 54 3 36E2 0/ 37 1 PHASE AT TORNEV Patented Aug. 25, 1942 ELECTROSTATIC SCANNINGRobert E. Graham, Bronx, N. Y., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationJune 4, 1941, Serial No. 396,521

22 Claims.

This invention relates to television transmitters and more particularlyto a television camera in which the scanning operation is accomplishedwithout the use of any moving material agent such as a focused electronbeam. v

The now common electronic scanning systems, while they constitute amarked improvement, especially for instantaneous image transmission,over the earlier systems employing moving optical elements arenevertheless limited from the practical standpoint by the necessity forthe provision of electron-optical systems for sharply focusing a beam orbeams of electrons onto a beam receiving target.

In order to escape from the restrictions imposed by the electron-opticalsystem, it has already been suggested to dispense entirely with theelectron beam and carry out the scanning operation by the movement alonga prescribed path, not of any material agent, but of a point or regionwhich is distinguished electrostatically from neighboring points. Forexample, it has been proposed to provide two plate-like conductorsspaced apart and supply them with'diiferent potentials to produce anelectrostatic field between them; to provide in this field a thirdconductor of elongated form and serving as a photo'- cathode on which animage is focused; and by changing the potential of the photocathode tocause a point or line separating parts of it from which photoelectronsare withdrawn from other parts from which they arenot withdrawn toprogress lengthwise of the photocathode in such fashion that theresulting current flowing from the photocathode to one of the plate-likecon- I v ductors bears a relation to the brightness of successive imagepoints.

electrodes are required, at least one and pref-' erably two of whichmust'be large as compared,

with the dimensions of the image line scanned,

while in other forms certain relations must obat the same orsubstantially the same potential as the collector anod e. This point orregion divides the first electrode into two parts over one of whichcollection of photoelectrons takes place,

while over the other it does not take place. This dividing point iscaused to progress along the length of the photocathode to scan theimage focused upon it by the provision of scanning voltages of suitablewave form on and between these electrodes. This results in the flow of acurrent in an external circuit connected to the collector which is ormay readily be converted into a vision signal for amplification andtransmission.

It is an object of this invention to provide improvements inelectrostatic scanning apparatus,

tain between 'the disposition of and coupling compact constructionemploying 'a minimum number of electrodes. The apparatus comprises twoextended electrodes, i. e., a photocathode on both of the type describedin the aforementioned application Serial No. 396,006 and of other types.Specifically it is an object to provide other and further means foreffecting the scanning movement of a neutral point or region along thelength of ,an extended photocathode, and in particular carriergenerators. It is a feature of the invention that the signal obtained inthe form ofa current from the collector is not a vision integral signalas in the arrangements shown in the aforementioned application SerialNo. 396,006 with the result that it becomes unnecessary to provide adifferentiating circuit to convert it into a conventional vision signal.On the other hand, the signal obtained from the collector does include acertain undesired component which it is impossible to remove on astraightforward frequency separation basis without degrading the desiredcomponent. Accordingly, it isa subsidiary object of the invention toprovide means for eliminating this undesired component.

It is another feature of the invention that by a simple adjustment theeffective aperture of the system may be altered at will and the signaldelivered by the apparatus may be made positive or nega ive, as desired;

While in a broad sense the invention may be useful in the scanning ofobject fields generally, it is especially applicable to the scanning ofimages borne by a moving film wherein scanning of successive image linesis carried out in accordance with the novel principles of the inventionwhile frame scanning, that is, scanning in a direction perpendicular tothe length of the image line, is accomplished by movement of the film.Accordingly, the following illustrative description is-directed in themain to preferred embodiments designed especially for line scanning. Itwill, be concluded .by a brief description of an embodiment suitable forfield scanning.

- 1 in which the movement of the scanning point is obtained byapplication to the photocathode of a novel combination of modulated andunmodulated high frequency carrier voltages;

Fig. 5 shows auxiliary apparatus which may be employed to obtain waveforms suitable for use with the apparatus of Figs. 4 and 6;

Fig. 6 shows a modification of Fig. 4;

Figs. 7 and 8 show arrangementsalternative to Figs. 1, 4 and 6,respectively, employing a discharge device containing auxiliaryelectrodes;

Fig. 9 shows an arrangement designed for scanning a surface asdistinguished from a line, the associated circuit being, for the sake ofillustration, the same as that of Fig. 1'; and

Fig. 10 shows a portion of the apparatus of Fig. 9 to an enlarged scale.

Referring now to Fig. 1 an elongated flexible member In is provided onwhich are recorded successive images of a field of view to betransmitted. For the sake of definiteness of illustration this member IDis shown as a transparent film, for example a motion picture film. Itmay be passed around sprocket or guide rollers H and anysuitable meansof a type well known in the art may be employed to maintain a part of itin-' termediate the guide rollers in a definite focal plane. Likewise,any suitable means may be employed to advance this portion of the filmin a whose unit impedance or resistivity is of an intermediate value,that is to say, it should be neither an insulator nor a good conductor,but should offer an impedance or resistance to the passage of electriccurrent such that it can sustain a comparatively large longitudinalvoltage drop even with the passage of a comparatively small current. Forexample, this element may be a wire of about 5 mils diameter and 1 inchin length and made of material such that with these dimensions its totalresistance as measured between its end terminals is of the order of 1megohm. If desired this element may be constructed in the form of atight spiral of small diameter in the manner well known in theincandescent filament art. It is important that the material of whichthe photocathode element -is constructed be uniform both as to its unitimpedance or resistivity vand its photoelectric properties so that thevoltage drop per unit length and the electron emission per unit ofillumination shall be the same throughout.

The collector anode 22 may be an elongated wire or strip of ordinaryconducting material such as a metal and may be disposed closeto andparallel with the photocathode within the envelope. The photocathodeelement and the collector anode may be supported and maintained incorrect parallel alignment by any suitable supporting means in a mannerwell known in the art.

Although the construction and arrangement as above described offercertain distinct advantages. an opposite arrangement may be employed, 1.e., one in which the photocathode is a highly conductive element whichtends to remain at a uniform potential throughout its length while thecollector anode offers a comparatively high impedance or resistance toelectric current and supports a longitudinal voltage drop. Thedescription in this specification will be directed principally to thepreferred arrangement though it is to be borne in mind that theinvention applies equally to the converse arrangement above brieflyalluded to. V

The pick-up device 20 may be disposed with the photocathode element 2|extending in a direction perpendicular to the length of the film Ill andthe line l5 of the film extending transversely of the film length andilluminated by the source l2 may be imaged upon the photocathode 2| sothat the amount of light falling upon each point of the photocathode isproportional to the translucency direction parallel with its length, forexample,

with a continuous uniform motion. Light from any suitable source, forexample an incandescent lamp filament I2 may be directed upon a filmframe 14, for example by a lens l3. frame [4 is thus evenly flooded withlight. A line I5 of the film frame I4 extending transversely of the filmlength may be imaged, for example by a lens IS, on the extendedphotocathode element of the novel pick-up device.

The pick-up device may be disposed in a position to be impinged by lightfrom the source passing through the transparent fihn It. This device maycomprise an evacuated envelope 20 containing two principal electrodes.The first electrode 2| is the photocathode and the second is a collectoranode 22. The photocathode may be an elongated narrow element, forexample, a wire or thin strip of electrically conductive material and,by reason of an appropriate surface treatment or otherwise, havingpronou nced photoelectric properties. The base material should be one ofthe corresponding point of the illuminated film line l5. Emission ofphotoelectrons will then take place from each point of the photocathode2| in proportion to the illumination of the corresponding points of theilluminated film line Ii in a well-known manner.

Operating potentials may be applied to the electrodes of the pick-updevice in various ways and may be of various forms. In the modificationof Fig. l a high frequency carrier voltage E1 derived from a generator4| is applied at constant amplitude to the end terminals of thephotocathode 2|, one of which is grounded. The output of this generatoris also fed to a modulator 43 which modulates the carrier envelope witha saw-tooth wave form at line scanning frequency. The resultingsaw-tooth modulated carrier voltage, designated E2, is applied to thecollector anode element 22. The collector anode 22 may be connectedthrough a bias battery E4, a resistor 23 through which the voltage E2 isapplied, to ground. At the same time it may be impressed instants oftime within a single cycle of the cara a loading resistor 36 from asuitable source, for

example, a battery 31, the negative terminal of which is grounded. Thesignal output from this tube appears across the loading resistor 36 andis delivered through a condenser 38 to a network of two separatebranches each containing certain elements for modifying the signal in aparticular manner as hereinafter fully described. A small condenser 46may be connected from the plate 35 to ground to by-pass undesiredcarrier frequency components of the tube output.

The internal connections and arrangement of the elements of the highfrequency carrier voltage generator 4| and of the device 43 whichmodulates the wave form of the carrier voltage with a saw-tooth linefrequency envelope may be of any desired type'since they form no part ofthe invention. Itis preferred that the carrier component of the outputof the modulator 43 which is applied to the collector anode 22 be inphase with the unmodulated carrier E1 applied to the photocathode 2|.This may likewise be provided for in any desired manner.

The frequency of the carrier voltages E1. and E2 applied to thephotocathode 2| and collector anode 22, respectively, should be high incomparison with the vision'frequencies to be produced. They may,therefore, properly be termed carrierfrequency voltage. The bias batteryE4 is included between collector anode and photo'- cathode for areasonlater explained. Its magnitud is preferably adjusted so that, inthe absence of the carrier voltages, the potential of the photocathode2| is just below the, collection threshold for a negative signal andjustabove for a positive signal.

The operation of the arrangement depicted in Fig. 1 for a negativesignal is as follows. The photocathode 2| being illuminated as abovedescribed, emission of photoelectrons takes place from the variouspoints thereof in proportion to the light incident thereon. Of thesephotoelectrons those emitted from points of the photocathode 2| whichare, at any particular instant of the carrier cycle, at negativepotentials with respect to the collector anode 22, are drawn to thecollector anode and caught by it, whereupon they return to thephotocathode through the external resistor 23' to produce a voltage dropacross the latter. On the other hand photoelectrons emitted from partsof the photocathode 2| which are at positive potentials with respect tothe collector anode 22 are repelled by it and fail to reach it andtherefore make no contribution to v the voltage 'drop across theexternal resistor 23.

Those parts of the photocathode from which collection takes place areseparated from those parts from which it does not take place by a pointat which, or a narrow region throughout which, the photocathodepotential is substantially equal to the collector potential or differsfrom it by a small amount equal to the'voltage of the bias battery E4.

The operation of the system of- Fig. 1 will be better understod byreference to th' diagram Fig. 2 in which the potentials of theelectrodes are plotted as functions of distance measured along the'photocathode element at two different rier voltage. Thus at one instant,-t1, the line Vci represents the potential of the photocathode whichincreases progressively along its length,

the left-hand end being taken as at zero or ground potential. Thecollectorv anode, a conductor, is at a uniform potential which, at theinstant t1, may be represented by the line Val. It will be observed thatunder these conditions parts of the photocathode to the left of thepoint of intersection B-are at lower potentials than the collector anodeand therefore active, whereas parts of the photocathode to the right ofthe intersection point 'B are at higher potentials than the collectorand therefore inactive. Thus,

at the instant t1 collection of photoelectrons istaking place from allparts of the photocathode between its left-hand end and the point B.This condition represented by the doted line Vs1 persists throughout onehalf cycle of the high frequency ca rrier voltage E1. During theimmediately ensuing half cycle of the carrier voltage the potentialconditions of the photocathode with respect to the collector arereversed, the part which was at a low potential and active during thefirst half cycle becoming inactive and vice versa. The new photocathodepotential is represented by the line V02 over which the potentialgradient is reversed and the new uniform potential of the collectoranode is represented by the horizontal line V3.2. Collection ofphotoelectrons now takes place from parts of the photocathode to theright of the intersection point B as indicated in the figure by thedottedline V's2 and not. from parts to the left. During the minuteinterval of time represented by one half cycle of the carrier voltagethe neutral point B has, of course, moved, but the amount of itsmovement in one carrier half cycle is imperceptible, or at least it issmall in comparison with the length of an elemental area of the lineimage focused on the photocathode. Thus over a full carrier cycle or aplurality of such cycles, emission take place from all points of thephotocathode to the collector except from the neutral Point B.

The potential variations of the collector in the course of themodulation cycle cause this neutral point Bto progress along the lengthof the photocathode at a constant speed under control of the saw-toothmodulation due to the modulator 43. Conditions at a later partof thesaw-tooth scanning cycle of the modulation envelope E2 are schematicallyrepresented on the diagram of Fig. 2 by the lines val and V112,respectively. It

will be observed that, due to the increase of the collector potential inthe course of the scanning cycle the point B has moved along thephotocathode element to a point B, where but for an invention, thisprocess continues untiLthe neutral point B has reached or slightlypassed the end of the line image focused on the photocathode element 2|After the neutral point has progressed over the full length of thephotocathode or that part of it on which the film line 5 is imaged, thesawtooth voltage envelope drops rapidly to its initial value to commencea new cycle, and the neutral region B flies back to its starting pointto commence the scanning of a new line. Meantime the film II! hasadvanced by the width of a single line so that as the neutral pointstartsits progress along the photocathode a slightly different part ofthe film will be imaged upon it. Successive repetitions of this processresult in complete scanning of the film image.

The amplitude of the carrier voltage is preferably set at acomparatively high value so that the carrier frequency voltage dropbetween collector and photocathode rises very rapidly on either side ofthe neutral point B to the value, indicated in Fig. 2 by the dottedlines Vs, which is necessary to insure saturatecfphotoelectroncollection from all points of the photooathode except a very narrowneutral region. Thus the effective cathodencollector potentialdifference is approximately uniform along the photocathode except for asharp dip in the vicinity of the neutral point. This sharp dip in thecathode-collector effective potential then corresponds to a rejection ofthe signal due to the light falling on or near the neutral point. As thecollector potential changes in the course of the saw-tooth scanningcycle the rejection point or barrier travels along the photocathode atthe prescribed sweep velocity, thus producing in the external collectorcircuit a signal of two components, first a signal representing theaggregate photoemission from the entire photocathode, and second, asignal of opposite sign, corresponding to rejection due to the barrierand proportional to the light distribution along the length of thephotocathode.

In the preceding explanation it has been assumed that the criticalpotential difference at which collection of photoelectrons commences iszero; that is to say no collection takes place when the collector is ata lower potential than the photocathode. This assumption is sometimesnot borne out by the facts. For example, collection may continue untilthe collector potential is one or two volts negative with respect to thePhotocathode. In this event, were it not for the bias E4, the collectionregion would extend beyond the nominal neutral point on one carrier halfcycle in one direction and would extend beyond it in the other directionon the next carrier half cycle. The result would be a'reduction of thedesired signal as compared with the undesired signal, and a consequentreduction in efliciency. The bias voltage E4 may therefore be employedto reduce the collector potential to the critical collection potentialso that the collection regions for successive half cycles just meet. Toogreat a negative biaswould produce a large gap between the collectionregions which would result in a loss of definition due to the increasedsize of the neutral region which is in effect the aperture of thesystem. Thus adjustment of the magnitude of this bias oifers aconvenient means for adjusting the aperture of the system.

These two signal components appear in the form of modulation envelopesof half cycle carrier frequency pulses, or, more precisely, since theneutral point difiers slightly from the collector potential,partial-cycle pulses. Since these pulses as so modulated have a netdirect current contribution over each carrier cycle instead of beingsine waves, the aggregate collector current will contain the two signalcomponents above mentioned both in terms of the usual vision frequenciesand also as modulation side-bands of the carrier frequency, so thateither the vision frequency signal itself or the carrier frequencysignal modulated with the vision signal maybe utilized as desired.Assuming that the vision signal itself is to be utilized, the carrierfrequency with its side-bands and harmonics may be filtered out by anyappropriate means, for example, a small by-pass condenser 46, which mayindeed be merely parasitic capacity between the plate 35 and ground. Thetwo signal components above described are impressed'together between thecathode 21 and control grid 25 of the tube 25.

If the same line image were to be continuously focused on' thephotocathode and repeatedly scanned, the first signal component abovementioned, namely the component representing the aggregate emission fromthe entire photocathode, would be harmless. However, in the usual casesuccessive line images are'to be scanned in which the aggregate light isnot constant from one line image to the next. In such case it isdesirable to dispose of this first component, leaving as the finaloutput signal of the apparatus only the second component abovementioned, namely, the one which represents the light distribution overthe whole image, from the beginning of the first scanning line to theend of the last.

The removal of the undesired component cannot be effected by an ordinaryseparation on the basis of frequency discrimination because the desiredvision signal itself contains certain essential components offrequencies equal to the frequencies of the undesired component, that isto' say the desired and undesired components overlap in frequencyspread. This fact, as well as an appreciation of a means and method bywhich the undesired component may be removed will be understood from thefollowing analysis which is based on the complete scanning of a singleimage frame or still picture, for example, a singl image frame M of thefilm l0. As is well known, the movement to be expected in ordinaryfields of view or the differences in aspect of successive film framesare of such a low order that the analysis on the basis of a stillpicture may be extended with no modifications of practical importance tothe scanning of moving object fields or of successive film frames.

Referring then to Fig. 3, the picture or image frame to be scanned maybe assumed to be set up on coordinate axes as indicated. The picture, ofwidth 2a and of height 2b, is to be swept parallel to the Y axis acrossa strip photocathode of length 211 (equal to the picture width) in the Xdirection and of height 2d (equal to the height of a single scanningline or strip) in the Y direction. At the same time the neutral point orregion which serves as a scanning spot is to be swept from the positiona to the position +a, that is, from end to end of the photocathodestrip. As explained above, collection of photoelectrons takes place overa complete carrier cycle from the whole strip except the neutral region.The latter may, therefore, be conceived of as a barrier equal in area tothe neutral region. For the sake of simplicity of analysis the barriermay be taken as a square of length and height both 211, i. e., of thesize and shape of an idealized elemental area of the picture.Furthermore it may be taken to be of complete opacity to photoelectronsand the carrier frequency collector currents will be assumed to beadequately by-passed or otherwise disposed of so that the carrierfrequenlcy variations in the collection of photoelectrons may bedisregarded and the collection taken as zero at the barrier and completeelsewhere.

These simplifying assumptions correspond to those generally made inscanning analysis and acknowledged "to be justified by the results.Without them the analysis would be so complex as ,to obscure its ownconclusions.

For any given position of the strip photocathode relative to the pictureand of the barrier relative to the strip, the coordinates of the centerpoint of the barrier may be designated as x and 11. Then the totalcollection of photoelectrons from the whole strip photocathode is given(neglecting constant factors) by:

' where L(:c,y)'=1ight distribution of projected picture, 1 =integrationvariables. y

In this equation the first term represents the aggregate collection fromthe whole strip. It would be the same if the barrier were not presnt.The second term represents the diminution of total collection due to thepresence of the where the As are complex constants.

Substituting this value of L into Equation 1 and integrating, thereis'obtained gmimm, Y(m,n) (exp)i1r (3) where Y(m,n) is an aperturedistortion factor.

By reference to the aforementioned treatment in the Bell SystemTechnical Journal it may be seen that in this Equation 3 the second termis of the form of an ordinary vision signal-obtained with conventionalscanning systems. It is therefore the desired signal. The first term,which is the undesired signal, is of the form obtained by putting m=o inthe second term. In other words, signal components representable byputting m=o are common to the desired and undesired signals. Thesecomponents are of importance since they are representative of images orscenes whose light values do not vary across the picture in the Xdirection but do vary in the the desired signal on the basis of aconventional frequency separation alone, and that more elaborate meansmust be employed to effect the sepa ration.

To this end the circuit arrangement of Fig. 1

is provided which permits the characteristics whichare common to thedesired and undesired signals to be taken advantage of. The wholesignal, including the desired and the undesired porlength 212 of theneutral region. (Fig. 3

The low frequency cut-off of the high-pass filter 52 and the highfrequency cut-off of the low-pass filter 53 are preferably set at thesame value which may advantageously be at or about one half the linescanning frequency of the sawtooth modulator 43. The attenuation ratioof the pad 54 may be set at a value equal to the ratio of the length ofa scanning line to the length, measured' along the line, of a singleelemental picture area, that is, the' ratio of the length 2a2d of theactive portion of the photocathode to the In accordance with present-daypractice this ratio may be about 400:1. The phase shifting device 55should be designed to alter the phase of the components passed by thefilter 53 by an amount which will place these components in the path 5|in phase opposition to the same components in the path 50. For example,if the phase rotation in the filters 52 and 53 andin the pad 54 bedisregarded, the phase shifter 55 may rotate the phase angle of the lowfrequency components through 180 degrees. A phase shifting device of anydesired type may be employed, for example, a single resistance-coupledtriode stage.

The operation of this circuit arrangement is as follows. Referring againto Equation 3, the latposed of a group of high frequency components Htions, is fed from the triode 26 into two separate paths, the one pathincluding a high-pass filter 52 and the other path 5| including intandem order a low frequency pass filter 53, an attenuator pad 54 and aphase shifting device 55. These two paths 50 and 5| are then broughttogether at common terminals and a group of low frequency components G;that IS V=H+G (5) and U, the undesired component, is (from (3) relatedto G by the equation Therefore the Equation 4 may again be written inwhich the components have been regrouped, the first term of theparenthesis containing essentially components below half line frequencywhile the second term contains essentially components above half linefrequency. Therefore when the signal as a whole is fed to paths 50 and5| in parallel, the components represented by the first term of (7) areexcluded by the filter 52, while passing the filter 53 and thecomponents represented by the second term are excluded by the filter 53.while passing the filter 52.

The first term components are then reduced in magnitude in the ratio (Tby the attenuator 54 and reversed in phase by the device 55, thus Theresulting signal, G, is then mixed with the signal H at the terminals 60to give (see Equation 4) and therefore the net signal at these terminalsmay be represented by Except for a reversal in phase of all componentsalike which is of no importance this is the desired vision signal.

The vision signals derived as above described may be amplified andtransmitted by wire or radio, by. carrier modulation or otherwise asdesired, to a receiver station where they may be reconstituted bysuitable apparatus as an image. If desired, the whole anode currentsignal including the undesired part may be transmitted to the receiverand the removal of the undesired component may be carried out at thereceiver end instead of the transmitter end. It is preferred, however,to effect the removal at the transmitter end.

Fig. 4 shows a further modification in which the scanning electromotiveforces are applied exclusively to the photocathode 2|, the potential ofthe collector anode remaining steady. In this case an unmodulatedcarrier voltage E1 and another carrier voltage E2 of like phase andfrequency but modulated with a modified saw-tooth scanning voltageareIapplied in series to the photocathode E1. The voltage E2 may bederived from a modulator l3 supplied with voltage from the carrierfrequency generator ll which also supplies the voltage-E1 to thephotocathode directly.

It will be understood that with modifications of a minor character theunmodulated carrier voltage E1 and the modulated carrier E2 may beinterchanged with reference to the photocathode and the collector anode,respectively.

The form of the modulating envelope may be calculated from Fig. 4 asfollows:

E1=unmodulated carrier Ez=modulated carrier Ze=photocathode impedance.Z1, Zz=dummy load impedances 45 and I then, measuring from the terminalconnected to the impedance 45 along the photocathode, the neutral pointwill appear at KZc, where Any desired means may be employed-to secure avoltage which variesinversely with time in the required manner. Forexample, an arrangement such as that disclosed in copending applicationSerial No. 342,601 filed June 27, 1940, or in United States Patent1,757,345, May 6, 1930, may be adapted to the present purpose. In thesedisclosures it is pointed out that currents having various desired waveforms may be generated by the use of a cathode beam tube in which abroad electron beam is projected on a suitably shaped target or on atarget disposed behind a suitably shaped aperture and swept over thetarget or aperture in accordance with a signal whose wave form it isdesired to modify. For the present purposes, if the aperture has theform. of a plot given by Equation 13, a current varying inversely withthe time in the required manner will flow in the target circuit when thebeam is deflected past the aperture at a uniform speed. This inversecurrent may be translated into a voltage of like wave form in anydesired manner, for example by allowing it to flow through a resistoracross which are connected the input terminals of a high impedance tube.The voltage appearing in the output circuit of this tube may be employedto modulate the carrier and the latter, as modulated, may be applied tothe photocathode to produce progressive movement of the neutral point asabove described.

Fig. 5 is a diagram showing the essentials of such a translating deviceand including a target 6| disposed behind an aperture in a plate 82 oneof whose sides is cut to follow the curve given by Equation 13 on thebasis of unity amplitude for E1. Deflection of an electron sheetoriginating at an extended cathode 63 past this aperture at uniformspeed will give a target current of the required wave form.

It will be apparent that with this arrangement the rate of change of thepotential difference between photocathode and collector is greater whenthe neutral point is near one end of the photocathode than when it isnear the other end. As a result, the region of saturated collection ofphotoelectrons is closer to the neutral point at one end of thephotocathode than at the other. In other words, the aperture effectvaries along the length of the photocathode and the received image willhave higher definition at one side than at the other. In someapplications this may be of advantage.

The signal current in the external circuit of the collector 22 andtherefore the voltage impressed on the input circuit of the triode 26contains the same two components as are derived in the apparatus ofFig. 1. These two components may be separated by a circuit identicalwith that above described, to deliver the desired vision signal at theoutput terminals 60. Furthermore, the high frequency carrier componentof the collector current may be by-passed to ground, for example, by theuse of a condenser 46, or may be disposed of in any other desiredmanner. v

Fig. 6 shows a modification of Fig. 1 in which the modulated carrier isapplied to the photocathode and the unmodulated carrier is applied tothe collector anode. In such case the carrier voltage E: may be derivedfrom'a carrier generator 4|", while the latter feeds a modulator 43"which supplies a modified wave form ensaw-tooth but rather an inversefunction of time, either of the form where a and b are constants.

In the practical application the modulating voltage need not becomeinfinite as these equations indicate that it must, when t is equal tozero or b respectively. This difliculty is most simply avoided by merelyproviding a photocathode of somewhat greater length than the line imagefocused on it so that the tag end fur nishes a marginal resistance. Ifpreferred, separate marginal resistors may be connected to the ends ofthe photocathode, either inside or outside of the envelope 20. It mayeasily be shown that inclusion of a marginal resistor or tag end of thephotocathode as a part of the circuit gives rise to a modified inversetime function,-

i. e., to the equation where Zc=tOta1 photocathode impedance;

Z1=impedance of marginal resistor or tag end;

Eo=minimum value of the saw-tooth voltage;

and

K is a constant.

It is to be noted both in Fig. 4 and Fig. 6 that since the saw-toothscanning generator is not applied directly to the triode 26, noauxiliary balancing generator such as shown in Fig. 1 and designated E3is required.

teraction between them. By suitable optical.

means, for example by the use of two lenses I6 and I6 as indicated, theline to be scanned is imaged in like manner on both photocathodes. Theexcitation of the principal photocathode and the scanning operationproceed as described above in connection with Fig. 1, but the auxiliaryphotocathode 2| is not scanned longitudinally. At the same time anothersignal similarto the undesired signal represented by the first term ofEquation 3 is derived from the auxiliary collector anode 22' and isimpressed on the auxiliary tube 26'. The impedanceelement 23 shouldpresent a substantial impedance value at .vision frequencies but mayhave a very low value at carrier frequencies. This serves to filteroutthe carrier voltage from the signal before it reaches the tube 26.The impedance element 23', on the other hand, need maintain its valueonlyup to about line frequency. The output is taken from the cathode oftube 26 and the anode of tube 26, so that corresponding components willbe in phase opposition. An adjustable element, for example, the cathoderesistor 28 of the tube 26 permits control of the magnitude of theoutput of the tube v26- to the value necessary to permit'it to cancelthe unde sired component in the tube 26 to leave only the desiredcomponent at the output terminals. In

order to simplify the drawings, the voltages E1 and E2 are shown asderived from separate generators, whichshould of course be synchronized.It is to be understood that they may likewise be derived from agenerator and modulator as in Fig. 1.

A tube with auxiliary electrodes as above described may also be employedto eliminate the undesired signal component in the circuit arrangementof Figs. 4 and 6. One such arrangement is shown in Fig. 8.

Although for the sake of particularity the desired signal has beendescribed throughout the preceding discussion as being of opposite signto the undesired signal, since it had its origin in a rejection ofphotoemission over the neutral region, it will be apparent that merelyby changing the bias potential E4 of the collector with respect to thephotocathode the collection regions for successive carrier half cyclescan be caused to overlap so that collection takes place over the neutralregion for both carrier polarities. this event the desired signal willbe of the same sign as the undesired signal and in other respects willbe as described above. This change of sign will be manifested as achange from a negative rejection signal to a positive double collectionsignal. It can be separated from the undesired signal by arrangementssimilar to those described above with modifications appropriate withrespect to the change of sign. For example,

in the separating circuit of Figs. 1, 4 and 6, the chief modificationwould consist in the omission of the phase shifter, secured by closingthe shortcircuiting switch, whereas in Figs. 7 and 8 no circuit changeis necessary, since these circuits operate by balancing out theundesired component whose signremains unchanged. I

Fig. 9 shows an arrangement by which the neutral point scanning of theinvention may be applied to the scanning of a whole surface image. asdistinguished'from a line image. In this figure the photocathode strip2| is bent back and forth to form a field net on which the image as awhole may be focused, as shown in greater detail in Fig, 10. Thecollector 22" is a plate mounted behind the photocathode net. Thescanning action, which may be secured in accordance with any of thecircuit arrangements hereinabove described, the arrangement of Fig. 1being shown merely by way of example, moves the neutral point along thecontinuous photocathode strip, effecting both horizontal and verticalscanning together without the necessityfor separate means for effectingline-by-line movement of the image across the photocathode strip. Afurther important advantage of this system is to be found in the factthat -it delivers a true vision signal combined with a signalrepresenting the aggregate illumination of the Whole image frame insteadof the aggregate illumination of a single line. This aggregateillumination signal may be utilized at the receiver as a direct currentbackground signal, thus eliminating the necessity of making separateprovision therefor.

Movement of the neutral point or region in the scanning operation mayalso be accomplshed by the use of a variable impedance element caused tovary according to a proper law, and suitably connected in circuit with'the photocathode or other electrode along whose length the neutral pointor region is to move. Such an arrangement maybe of various types, someof which form the subject-matter of copending application Serial No.396,522, filed June 4, 1941.

The fact that the invention has been illustrated and described in termsof an embodiment wherein one of the electrodes supports a longitudinalvoltage gradient is not to be taken as restrictive. The carrierexcitation of the invention may also be applied with advantage to otherapparatus in which, for example, the electrodes are all at uniformpotentials and the boundary or barrier region is caused to movelongitudinally of one of them by means of suitably modulated carrierfrequency potential differences between them. When in such anarrangement an undesired signal arises of the type dealt with above, it

likewise may be removed by any of the above-described arrangementsadapted to effect such removal, independently of the particular natureof the apparatus in which the undesired signal may originate.

What is claimed is:

1. Image signal translating apparatus which comprises an extendedphotocathode element disposed in position to be illuminated in a mannersuch as to cause electron emission from each elemental area thereof inaccordance with the illumination of said elemental area, at least oneextended anode element disposed to intercept electrons emitted by saidphotocathode, means for causing alternate collection and rejection by aan anode of 'photoelectrons from areas of said photocathode extendingfrom one end thereof to a boundary region, said collections andrejections alternating at a high carrier frequency means for causingsaid boundary region to progress longitudinally of said photocathodecyclically at a line-scanning frequency, and means for drawing signalcurrents from said anode.

-2. Image signal translating apparatus which comprises an extendedphotocathode element disposed in position to be illuminated in a mannersuch as to cause electron emission from each elemental area thereof inaccordance with the illumination of said elemental area, at least oneextended anode element disposed to intercept electrons emitted by saidphotocathode, means for causing collection by an anode of electrons fromareas located in opposite parts of said photocathode in alternation,which parts are separated from each other by a boundary region, meansfor causing said boundary region to progress longitudinally of saidphotocathode, and means for drawing signal currents from said anode.

3. Image signal translating apparatus which comprises an extendedphotocathode element disposed in position to be illuminated in a mannersuch as to cause electron emission from each elemental area thereof inaccordance with the illumination of saidelemental area, at least oneextended anode element disposed to intercept electrons emitted by saidphotocathode, means for causing collection by an anode of electrons fromareas located in opposite parts of said photocathode in alternation,which oppositely located areas have a boundary region in common, saidcollections alternating at a high carrier frequency, means for causingsaid boundary region to progress longitudinally of said photocathode,and means for drawing signal currents from said anode.

4. Image signal translating apparatus which comprises an extendedphotocathode element disposed in position to be illuminated in a mannersuch as to cause electron emission from each elemental area thereof inaccordance with the anode element disposed to intercept electronsemitted by said photocathode, means for producing an electrostatic fieldbetween said two elements, said field having one polarity over a part ofone of said elements extending from one end of said last-named elementto a boundary region and an opposite polarity over a part extending fromthe other end of said last-named element to said boundary region, meansfor reversing said electrostatic field at a high frequency, means forcausing said boundary region to progress longitudinally of saidlast-named element, and means for drawing signal currents from saidanode.

5. Image signal translating apparatus which comprises an extendedphotocathode element disposed in position to be illuminated in a mannersuch as to cause electron emission from each elemental area thereof inaccordance with the illumination of said elemental area, an anodedisposed to intercept electrons emitted by said photocathode, means forproducing an electrostatic field between said anode and saidphotocathode.

- said field having one polarity over a part of said photocathodeextending from one end thereof to a boundary region and an oppositepolarity over parts extending from the other end of said photocathode tosaid boundary region, means for reversing said electrostatic field at ahigh frequency, means for causing said boundary region to progresslongitudinally of said photocathode, and means for drawing signalcurrents from said anode.

6. Image signal translating apparatus which comprises an extendedphotocathode element disposed in position to be illuminated in a mannersuch asto cause electron emission from each elemental area thereof inaccordance with the illumination of, said elemental area, at least oneextended anode element disposed to intercept electrons emitted by saidphotocathode, means for causing collection by an anode of electrons fromareas located in opposite parts of said photocathode in alternation,which areas meet in a boundary region, said collections alternating at ahigh carrier frequency, means for causing said boundary region toprogress longitudinally of said photocathode, means for drawing signalcurrents from said anode, said signal currents containing a desiredvision component and an undesired component, said two components havingat least one frequency in common, and means for removing said undesiredcomponent.

7. Apparatus as defined in claim 6 wherein said means for removing anundesired component comprises an auxiliary photocathode similar to saidprincipal photocathode and similarly disposed to be similarlyilluminated, an auxiliary anode similarto said principal anode andsimilarly disposed to intercept electrons emitted from said auxiliaryphotocathode, means for drawing auxiliary signal currents from saidauxiliary anode, and means for balancing said auxiliary signal currentsagainst said principal signal currents.

8. Apparatus as defined in claim 6 wherein said means for removing anundesired component comprises a network of two parallel branches intoboth of which said entire signal currents are fed, one of said branchescontaining means for passing only oscillations of all frequencies abovesaid common frequency and blocking said common frequency, the other ofsaid branches containing means for passing said common frequencyand'frequencies below said common frequency and blocking all otherfreillumination of said elemental area, an extended quencies, means inone of said branches for adto intercept electrons emitted from saidphotocathode, means for applying a carrier frequency voltage of constanthigh frequency to the end terminals of one of said elements to produce avoltage gradient along its length, means for applying another voltagebetween said two elements, means for varying one of said voltages tosweep a region at which the potentials of said elements aresubstantially alike along the length of said gradient-supportingelement, means for drawing current from said collector element, andmeans for removing undesired components from said current.

10. Apparatus as defined in claim 9 wherein the voltage applied betweenthe two elements is a carrier voltage of frequency equal to that of saidfirst-named carrier voltage and of related phase, amplitude-modulatedwith a saw-tooth voltage of line-scanning frequency.

11. Apparatus as defined in claim 9 wherein the first-named voltage iscompounded of.two synchronized high frequency carrier voltages, the onebeing unmodulated and the other being modulated by a voltage of a waveform such as to cause said equipotential region to be swept at constantspeed, and the second-named voltage is l a steady bias.

12. Image signal translating apparatus which comprises an extendedphotocathode element disposed to be illuminated in a manner such as tocause electron emission from each elemental area thereof in accordancewith the illumination of said elemental area, an extended anode disposedto intercept electrons emitted from said photocathode, means forapplying a carrier frequency voltage of constant high frequency to theend terminals of one of said elements to produce a voltage gradientalong its length, which gradient is spatially uniform but alternates ata high frequency, means for varying the amplitude of said voltage tosweep a region at which the potentials of said elements aresubstantially alike along the length of said gradient-supportingelement, means for drawing current from said collector element, andmeans for removing undesired components from said current.

13. Image signal translating apparatus which comprises an extendedphotocathode element disposed to be illuminated in a manner such as tocause emission of photo-electrons from various points of said element inaccordance with the illumination of said points, an anode elementdisposed to intercept electrons emitted from said photocathode, meansfor causing said anode to collect'electrons emitted from all points ofsaid photocathode except a barrier region, means for causing movement ofsaid barrier region along said photocathode, and means for utilizingcurrent drawn from said anode.

14. Image signal, translating apparatus which comprises an extendedphotocathode element dis posed to be.illuminated in a manner such as tocause emission of photo-electrons from various tion of said points, ananode element disposed to intercept. electrons emitted from saidphotocathode, means for causing said anode to collect electrons from allpoints of said photocathode except a barrier region, means for causingrepeated movement of said barrier region along said photocathode at aline-scanning frequency, means for drawing from said anode a signalcurrent containing a desired vision component and an undesired componentcontaining frequencies present in said desired component, and means foreliminating said undesired component.

15. In combination with image signal translating apparatus characterizedby an efiectively composite scanning aperture, one component aperturebeing of the length of a scanning line and moving in a frame scanningdirection, the other component aperture being comparable in size with anelemental area and moving along said first component aperture in a linescanning direction, said apparatus being adapted to deliver a desiredsignal derived from said second component aperture and an undesiredsignal derived from said first component aperture, said two signalshaving at least one frequency component in common, means for separatingsaid undesired component from said desired component which comprisesmeans for excluding said common component from said desired signal,means for adjusting the amplitude of undesired signal to bring thecommon component thereof to the level of the common component in saiddesired signal, and means for reinserting said common component asmodified into said desired signal as modified, to produce said desiredsignal unmodified. 16. In image signaling apparatus, in combination witha circuit arrangement carrying a desired vision signal having aplurality of frequency points thereof in accordance with the illumlna-76 components and an undesired signalhaving at least one frequencycomponent in common with said desired signal, apparatus for separatingsaid undesired signal from'said desired signal which comprises a networkof two parallel branches, one of said branches including a high-passfilter constructed to pass all components of said' desired signal exceptsaid common component, the other of said branches including a low-passfilter arranged to include all components of said undesired signal andno components of said desired signal other than said common components,one of said branches including means for adjusting the amplitude of saidcommon component of said undesired signal in a stipulated amount, and

means for mixing the signals of one of said branches as modified by thecircuit elements of saidbranch with the signals of said pther branch asmodified by the circuit elements of said other branch. I

'17. Image signal translating apparatus which com rises two extendedphotocathode elements each of which is disposed to beilluminatedin amanner such as to' cause emission of photoelectrons from various pointsthereof in accordance with the illumination of said points, an anodeelement disposed to intercept electrons emitted from one of saidphotocathodes, another anode element disposed to intercept electronsemitted from the other of said photocathodes, means for causing one ofsaid anodes to collect electrons from all points of its associatedphotocathode except a barrier region, means for causing the other ofsaid anodes to collect electrons from all points of its associatedphotocathode, means for causing movement of said barrier region alongsaid photocathode, means for drawing current from each of said anodes,and means for balancing said anode currents against each other toprovide a signal related to the movement and illumination of saidbarrier.

18. Image signal translating apparatus which comprises a photocathode,means for imaging a part of a field of view on said photocathode tocause electron emission from various points thereof in accordance withthe light-tone values of corresponding points of said image, an anodedisposed to intercept electrons emitted from said photocathode, ageneratorof carrier frequency oscillations, means including saidgenerator for causing collection of photoelectrons emitted from allpoints of said cathode located on one side of a neutral point duringcarrier oscillation half cycles of one polarity and for causingcollection of photoelectrons emitted from all points of said points ofsaid image, means for causing said anode to collect electrons emittedfrom all points of said photocathode except a barrier region, means forcausing movement of said barrier region along said net-like element fromend to end thereof, and means for utilizing current drawn from saidanode.

21. Electrostatic scanning apparatus which comprises an extendedphotocathode disposed in position to have an image of a line of a fieldof view projected upon it in a manner such as to cause electron emissionfrom each point of said photocathode in dependence upon the light-tonevalue of a corresponding point of said image, an anode disposed tointercept electrons emitted from said photocathode, means for creatingon said photocathode a region over which collection of photoelectronsdiffers in amount from said collection elsewhere on said photocathode,the

cathode located on the other side of said neutral length of said regiondefining an effective scanpoint during carrier oscillation half cyclesof the other polarity, means for causing movement or said neutral pointalong said photocathode at a line scanning frequency, and means forutilizingcurrent drawn from said anode.

19. Image signal translating apparatus which comprlstes an extendedphotocathode element disposed to be illuminated in a manner such as tocause electron. emission from each elemental area thereof in accordancewith the illumination of 80 said elementa larea, an extended anodedisposed to intercept electrons emitted from said photocathode, meansincluding a constant carrier frequency generator for producing a uniformvoltage gradient along the length of one of said elements, whichgradient oscillates in polarity at the carrier irequency, means forsweeping a region at which the potentials of said elements are alikealong the length of said gradient-supporting element, means for drawingcurrent from said collector element, and means for removing undesiredcomponents from said current. 20. Image signal translating apparatuswhic comprises an extended photocathode element, an extended anodeelement disposed to intercept electrons emitted from said photocathode,one of said elements being arranged in the form of a net extended in twodimensions, the other of said elements being plate-like in form, meansfor imaging a field of view on said photocathode to cause electronemission from various points thereof in accordance with the brightnessof corresponding ning aperture, means for causing said region toprogress along said photocathode to scan said image, means fortranslating currents drawn from said anode into vision signals, andmeans for adjusting the length of said region to provide an effectiveaperture of desired size;

22. In electrostatic image translation apparatus, an extended linealresistance photocathode element disposed in position to be illuminatedin a manner such as to cause electron emission from each elemental areathereof in accordance with the illumination of said elemental area, anextended lineal conductive anode element disposed parallel with andclose to said photocathode element, means for causing a current to flowthrough said photocathode element from end to end thereof to establish auniform voltage gradient therealong, means for reversing said current ata constant high frequency to cause reversal of the direction of saidgradient at said high frequency, means for maintaining the potential ofsaid anode at the potential of a point of said photocathode which pointis intermediate the ends of said photocathode, said potential beingintermediate the peak values of the potentials of the ends of saidphotocathode, means for sweeping said equipotential points lengthwise ofsaid elements at a line-scanning frequency, and means for utilizingcurrents drawn from said anode.

- ROBERT E. GRAHAM.

